MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

SECTION 1. BIOLOGICAL BASES OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.3. Biology of Individual Development

CHARACTERISTICS OF THE Prenatal period of Human Development, Critical Periods

In human prenatal (before birth) development, three periods are distinguished: 1) initial (pre-embryonic); 2) embryonic; 3) fetal. The initial period lasts from Fertilization throughout the first week of Pregnancy. The Embryonic period (weeks 2-8) is termed the period of Organogenesis. During this period, the three germ layers (ectoderm, endoderm, and mesoderm) give rise to all Tissues and Organ Systems. The processes of organogenesis are highly sensitive to the damaging effects of genetic and environmental factors. Most major developmental anomalies arise during this period. By the end of this period, the embryo becomes a fetus with distinctively human body contours. The fetal period (from the 9th week of pregnancy until birth) is characterized by rapid body growth and the maturation of organ systems.

Critical periods are intervals when the embryo is most susceptible to adverse environmental factors. The Concept of Critical developmental periods was introduced in 1921 by C. Stockard and further developed by P.G. Svetlov. The following critical periods are distinguished in human development: 1) implantation (days 6-7 post-fertilization); 2) placentation (the end of the 2nd week of pregnancy); and 3) the perinatal period (childbirth). The latter critical period is associated with drastic Changes in the newborn's living conditions and the reorganization of all body systems (alterations in Circulation, gas exchange, and Nutrition). In addition, critical periods for The Development of individual Organs have been established throughout the embryonic life span.

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Fig. 1.87. Puffs in Chromosomes (after Dubinin):

microphotograph of a chromosome from a Drosophila salivary gland Cell showing three large puffs (swellings) at this stage; black dots indicate the sites of radioactive uridine incorporation